Our previous studies suggest that cyclic 3',5'-adenosine monophosphate (cAMP) plays a regulatory role in milk production inhibiting lactose synthesis. Last year we investigated the partial inhibition by exogenous cAMP or adenylate cyclase stimulation. A potent cAMP analog, p-chlorophenylthio-cAMP, showed the same 30-40% inhibition as cAMP but at 1/100th the dosage, suggesting the partial inhibition reflects more than poor cAMP transport into the cells. Adenylate cyclase stimulation was investigated by measuring lactose production in the presence of 0.01-10 microgram/ml cholera toxin which maximally stimulates cyclase. However, the toxin had no effect on lactose production over a 2 1/2 hr interval suggesting either lack of cyclase activity or an inactive receptor. Previous studies showing inhibition of lactose secretion by colchicine were extended with dose-response studies of colchicine, vinblastine and lumichochicine. The results suggest that microtubules play an important role in lactose secretion but not in its synthesis. Finally we showed that the cAMP-induced growth enhancement of the "milk" secreting rat mammary tumor, R3230AC, was due both to increased cell proliferation and cell hypertrophy. During the third year of the project we will: (1) examine the mechanism of cAMP inhibition of lactose synthesis by measuring several components of the cyclic nucleotide system during pregnancy and lactation as well as the lactose synthetic pathway; (2) follow tubulin changes during pregnancy and lactation. BIBLIOGRAPHIC REFERENCES: Klein, D.M. and Loizzi, R.F. Enhancement R3230AC rat mammary tumor growth and cellular differentiation by dibutyryl cyclic adenosine monophosphate. J. Natl. Cancer Inst. 58: 813-818, 1977. Guerin, M. Anita and Loizzi, R.F. Effects of microtubule-altering drugs on lactose synthesis and secretion by lactating guinea pig mammary gland. Fed. Proc. 36: 343, 1977. (Abstract)